Conventionally, as a method for connecting end portions of pipes such as oil well pipes to each other, a conventional method has been to form a thread part (external thread part) at each of outer peripheral surfaces of the end portions of the pipes to provide threaded pipes and fasten each of the respective thread parts (external thread parts) of a pair of the threaded pipes to a joint (box joint) with a thread part (internal thread part) formed at an inner peripheral surface thereof to connect the end portions of the pipes.
A low dimensional accuracy of the thread parts formed at the end portions of the pipes may loosen a fastened state to the joints, resulting in the pipes being disconnected and coming off or a fluid flowing inside the pipes leaking to the outside. In particular, requirements for dimensional accuracies or quality assurance levels of thread parts of oil well pipes are becoming strict year by year along with an increase in harshness of the oil well environments in recent years.
FIGS. 1A and 1B are cross-sectional diagrams schematically illustrating an example of an end portion shape of an oil well pipe. FIG. 1A is a cross-sectional view of the entire end portion, and FIG. 1B is an enlarged view of the circular region indicated by symbol X in FIG. 1A.
As illustrated in FIGS. 1A and 1B, an end portion of an oil well pipe P includes a thread part P3 provided with a thread ridge P1 and a thread groove P2, a parallel part P5 adjacent to the thread part P3, the parallel part P5 being provided on the pipe end face E side relative to the thread part P3, and a seal part P4 adjacent to the parallel part P5, the seal part P4 being provided on the pipe end face E side relative to the parallel part P5.
Along with an increase in harshness of the oil well environments in recent years, pipes including a thread part P3 formed therein are often used for oil well pipes P. The thread part P3 includes a pair of flank faces P6 defining a thread ridge P1 of the thread part P3 (surfaces connecting a top part P11 of the thread ridge P1 and bottom parts P21 of the thread groove P2), and from among the pair of flank faces P6, a flank face P6 facing away from the pipe end face E side (flank face P6 on the side on which a load against a tensile force in a thread axis (center axis of the thread part P3) “A” direction is imposed when connecting pipe end portions) is a flank face inclined so as to be closer to the pipe end face E side from the top part P11 of the thread ridge P1 toward the bottom part P21 of the thread groove P2 (hereinafter referred to as “hook-like flank face P6h”).
Conventionally, for evaluation of an angle (angle formed with a line N perpendicular to the thread axis A) α of a flank face P6 or a curvature of a thread root R part (part in which a flank face P6 and a bottom part P21 of a thread groove P2 cross) P7, a mold is taken of the thread part P3 using, e.g., silicon rubber. Subsequently, the mold is cut into cross-sections such that the angle of the flank face P6 or the curvature of the thread root R part P7 can be evaluated, and an enlarged shadow of the cross-section is projected onto a transparent film using a projector. On the transparent film, a tolerance for the cross-section according to a tolerance of the angle of the flank face P6 or a tolerance of the curvature of the thread root R part P7 is outlined, and whether or not an edge of the shadow falls within the tolerance for the cross-section is checked visually to determine the acceptability of the angle of the flank face P6 or the curvature of the thread root R part P7.
Also, an inspection similar to that of the aforementioned thread part P case may be conducted for a curvature of a front edge R part (part where a shoulder face P41 and a seal face P42 of a seal part P4 cross) P43 of the seal part P4.
The inspection such as mentioned above requires a great deal of time and effort in, e.g., taking a mold of the thread part P3 or the seal part P4 and cutting the mold into cross-sections. Thus, a sampling inspection, such as an inspection being performed only for first and last oil well pipes P in a same production lot, is inevitable because of difficulty to conduct a total inspection.
Furthermore, the acceptability is determined merely based on a comparison with the tolerance, and thus, a quantitative evaluation in shape of the thread part P3 or the seal part P4 is difficult.
In order to solve such problems, Patent Literatures 1 and 2 each propose a method in which light is radiated in parallel to a thread groove P2 to detect light passing through a thread part P3 to measure an external shape of the thread part P3 (which is a recessed and projecting shape of a surface of the thread part P3 and is referred to as “thread profile”) (light projection method). If the thread profile of the thread part P3 can be measured with good accuracy using the light projection method, an angle of a flank face P6 and a curvature of a thread root R part P7 could be calculated with good accuracy.
However, since the thread profile has a curved line, a flank face P6 may fall in the shadow of a ridge line of the thread ridge P1, in which case it is impossible to accurately detect the flank face P6 using the aforementioned light projection method in which parallel light is detected. In particular, if the flank face P6 is a hook-like flank face P6h, an error caused as a result of the hook-like flank face P6h falling in the shadow of the ridge line of the thread ridge P1 cannot be ignored.
Therefore, as described in Patent Literature 3, the present applicants propose a method of measuring a shape of a flank face P6 of a thread part P3 using a contact probe.
However, in the method described in Patent Literature 3, a contact probe is sequentially moved and a spherical contactor attached to a front edge thereof is brought into contact with the flank face P6 to perform measurement, inevitably resulting in long measurement time and an insufficient number of measurement points. Thus, an angle of the flank face P6 may not be measured accurately.
Also, since the contactor has predetermined dimensions, it is also difficult to measure a curvature of a thread root R part P7 with good accuracy.
Also, Patent Literature 3 described above and Patent Literature 4 propose a method of detecting a thread axis.